Irinotecan (CPT-11) has been used as a first line drug in the treatment of colorectal cancer, the third most commonly diagnosed cancer in the world. However, its efficacy and safety is compromised because of severe neutropenia and late diarrhea, the side-effects resulting from CPT-11 bioactivation and subsequent metabolism. CPT-11 is a prodrug that is hydrolyzed by carboxylesterase to the active topoisomerase 1 inhibitor, SN-38. Inactivation and detoxification occurs primarily by UGT1A1 catalyzed glucuronidation to form SN-38 glucuronide (SN-38G). SN-38G is excreted via the biliary ducts into the gastrointestinal (GI) tract, where it serves as a substrate for bacterial ?-glucuronidase enzymes produced by microflora. Free SN-38 is absorbed into the GI tract through passive diffusion or active transport. In 2005 and 2010, the FDA updated the label for CPT-11 regarding the heightened risk of serious side effects for patients that inherit the UGT1A1*28 allele and exhibit reduced expression of UGT1A1. However, several recent clinical studies summarize a trend of improved survival and statistically significant higher tumor response rates among individuals that are homozygous for the UGT1A1*28 allele. It is important to fully understand the association between SN-38 directed glucuronidation and the pending intestinal tissue damage resulting from CPT-11 therapy. Recently, we have generated an Ugt1 conditional knockout mouse model targeting deletion of the Ugt1 locus specifically in liver tissue (Ugt1?Hep mice). Surprisingly, even with the complete absence of the hepatic Ugt1 locus and all UGT1A proteins, Ugt1DHep mice are quite resistant towards CPT-11 treatment and the ensuing intestinal tract toxicity. Thus, we hypothesize that extrahepatic expression of the UGT1A proteins contributes to the detoxification of SN-38, which can be further confirmed in mice following gastrointestinal tract deletion of the Ugt1 locus (Ugt1?GI mice). Importantly, we speculate that GI tract directed control of the Ugt1 locus in mice can be leveraged to directly examine the contribution of this tissue towards SN-38 elicited intestinal damage. To examine the contribution of tissue specific expression of the Ugt1 locus in mice towards CPT-11 initiated intestinal damage, we will perform the following experiments. Specific Aim 1 will be directed to determine the impact of organ specific deletion of the Ugt1 locus in CPT-11 delayed diarrhea. Studies outlined under this aim will explore differences in CPT-11 induced intestinal toxicity in both Ugt1?Hep and Ugt1?GI mice. Specific Aim 2 will examine the impact of inducing selectively intestinal UGT1A1 expression and its enhanced contribution towards protecting intestinal tissue from the toxic insult generated from increasing concentrations of SN-38. With novel animal models recently developed in our laboratory, these studies will identify the role of hepatic and intestinal glucuronidation towards the etiology of CPT-11 induced intestinal toxicity. These models can be exploited to identify therapeutics that will improve the therapeutic index and efficacy associated with CPT-11 treatment.